As the apparatus for automatic control of the concentration of a chemical treating solution, there has been known an apparatus which analyzes the concentration of the predetermined component consumable by the chemical treatment in a chemical treating solution and, when the concentration is detected to be lower than the command concentration based on the analyzed result, automatically supplies a replenisher in a required amount in order to restore the concentration to the predetermined concentration. Another apparatus has also been known which not only computes or calculates the consumed amount per unit time of a component in a chemical treating solution on the basis of the surface area of material to be chemically treated to supply the loss of the component, but also analyzes the component in the chemical treating solution continuously to control the supply amount of the component based on the degree of the concentration (Japanese Patent Application Laid-open No. 199069/1986).
In the former apparatus described above, however, the replenisher to be supplied is added in a required amount after sampling the chemical treating solution, analyzing the concentration of the predetermined component in the chemical treating solution, and further detecting that the concentration has become lower than the predetermined concentration from the analyzed result. Therefore, a time lag from sampling of the chemical treating solution to addition of the replenisher occurs, which is a problem of the apparatus.
That is, for example, in case where electroless copper plating is carried out as a chemical treatment, sodium hydroxide and formalin as a reducing agent in a plating solution as a chemical treating solution reduce copper ion into metal copper to deposit copper plating. As a result, the concentrations of the three components decrease gradually while progressing with the plating reaction. Accordingly, in the above former apparatus, each concentration of the three components is analyzed independently and then loss of each component is supplied based on the analyzed result in electroless copper plating. In this case, if the concentration analysis of each component is carried out by a conventional analytical method in which the concentration of sodium hydroxide is firstly determined by titration, next the copper concentration is determined by absorption spectroscopy, and finally the formalin concentration is determined by titration, it would take as much as 15 to 30 minutes for the analysis. Therefore, the time lag over 15 to 30 minutes would occur in the period from sampling of a plating solution as a chemical treating solution to addition of the replenisher.
Therefore, in the above former apparatus, the analyzed result and the actual concentration at the replenisher-adding point of time are sometimes different largely from each other, which results in a problem that the concentration of the predetermined component in the chemical treating solution can not be accurately controlled to the command concentration by addition of replenisher. Such a problem might cause a serious disturbance on the precise control of concentration of the chemical treating solution to within a narrow range as possible and on the uniformity of physical properties of a plated film formed on material to be treated by the chemical treatment and treating rate. On the other hand, the above latter apparatus is one which calculates the consumed amount per unit time of the predetermined component in the chemical treating solution based on the deposition amount per unit time of the treated film calculated on the basis of the surface area of the material to be treated, under certain chemical treating solution compositions and certain treatment conditions, and then supplies the predetermined component in an amount corresponding to the consumption into the chemical treating solution. Therefore, in that apparatus, the time lag as observed in the above former apparatus does not occur. Furthermore, in case where the concentration of the chemical treating solution is maintained at the predetermined control concentration and it is needless to adjust the amount of replenisher, constant concentration control constant to some extent is possible by using the apparatus.
However, in such apparatus, it is required to analyze the concentration of the predetermined component in order to confirm whether the concentration of the chemical treating solution is maintained at the predetermined control concentration or not. When it is confirmed that the concentration of the chemical treating solution is not maintained at the predetermined control concentration, a control such as stopping to supply the replenisher, reduction in supply amount of the replenisher or increase in supply amount of the replenisher, or a control in which another kind of replenisher is added should be carried out.
Accordingly, in such controls, after all, the time lag from analysis of the chemical treating solution to addition of replenisher occurs like the above former apparatus. Furthermore, there also occur another problem that the concentration of the chemical treating solution can not be maintained within the predetermined narrow range and that physical properties of plated film formed on material to be treated by the chemical treatment and the treating rate can not be make uniform.
In addition, such apparatus is useful only under certain chemical treating solution compositions and treatment conditions. Therefore, when the treatment conditions such as surface area of material to be treated are different, the control rule to be employed should be changed depending on the conditions, which result in a problem that the control is troublesome.
Furthermore, the conventional apparatuses described above assume their use to be in the case of relatively low chemical treating rates, for example, where it takes about 20 hours for treatment, in which change in concentration of each component is small and the supply amount accompanied by the change is also small. Therefore, in this case, such apparatuses can be utilized usefully to some extent. However, in case of relatively high chemical treating rates, for example, where it takes about 2 hours for treatment, the reaction of the chemical treatment is fast and the change in concentration is large. In this case, such apparatuses could not be utilized at all.
That is, for example, in the conventional electroless copper plating in which the plating time is by 20 hours, the plating deposition rate is 1 to 2 .mu.m per 1 hour, which is low speed. In this case, the time lag owing to analyzing time of plating solution is not so serious problem. On the other hand, in a high speed electroless copper plating in which the plating time is 2 hours, the plating deposition rate is 15 to 20 .mu.m per 1 hour, which is high speed. In this case, the analyzing time of plating solution (15 to 30 minutes) becomes a serious problem.
The present invention is accomplished in view of these problems. The present invention provides a method for detecting concentration of a chemical treating solution, in which the concentration of the predetermined component in the chemical treating solution at the current point of time can be accurately detected constantly without influence by analyzing time; and provides an automatic control method for a chemical treating solution using said concentration detecting apparatus, which is suitable for controlling the concentration of the chemical treating solution.